1. Field of the Invention
This invention relates to the use of amorphous magnetic alloys having high magnetostrictive effects and more particularly, to stress-sensitive sensors, such as oil pressure and torque sensors, which make use of the alloys and can stand use under severe conditions of high stress or in a condition full of oil or oil mist.
2. Description of the Prior Art
A variety of amorphous alloys have been produced by rapid quenching methods, among which iron-base amorphous alloys are considered to have wide utility as magnetic materials because of the high saturation magnetic flux density and the great magnetostrictive effect. For instance, attempts have been made to apply the alloys as cores for transformers, and as diaphragms of magnetostrictive sensors such as pressure sensors. One of the applications which have now been reduced to practice, is an oil pressure sensor for detecting high oil pressure in systems. In this sensor, an amorphous magnetic alloy in the form of a diaphragm, which is placed in magnetic field, is distorted or strained by application of oil pressure. This distortion causes permeability of the diaphragm to vary. This variation of the permeability is detected by a suitable means as a variation of magnetic flux or inductance, followed by conversion into an oil pressure. Thus, the applied pressure can be measured. The amorphous magnetic alloys used for these purposes are employed under so severe conditions that they are brought into direct contact with oil and suffer high pressure. In this sense, the alloys should be soft magnetic materials which have high mechanical strength, good resistance to corrosion, and large saturation magnetostriction. Fe-Si-B alloys are known as soft magnetic materials which have high mechanical strength and large saturation magnetostriction. For example, alloys having compositions of Fe.sub.78 Si.sub.10 B.sub.12 and Fe.sub.60-90 Si.sub.0.1-15 B.sub.10-30 have been proposed, as pressure sensors, in Japanese Laid-open application No. 49-91014. However, when applied as an oil pressure sensor, the alloys tend to be highly oxidized because oxygen gas, water, and oxides in oils come into contact with the alloys under high pressure. This results in lowerings of mechanical strength and magnetic characteristics, which will bring about breakage of the sensor or at least deteriorate the sensor with time.
Another problem involved in the sensor using the Fe-Si-B alloys is that expensive boron is contained in an amount as great as 10 to 30 atomic percent, and thus the cost of the sensor itself becomes very high, thus permitting only a limited use. In order to apply amorphous magnetic alloys as oil pressure or other sensors, it is believed essential to use inexpensive amorphous alloys having good corrosion resistance.
In order to improve the corrosion resistance, it may be effective to substitute part of iron in the alloys with nickel or chromium. However, when nickel or chromium is added to the alloys, the saturation magnetostriction lowers considerably and thus the alloy is not suited for a sensor because of the reduction of the magnetostrictive effect of the alloy.
For the manufacture of amorphous magnetic alloys at low costs, it is convenient to lower the concentration of the most expensive boron in the alloy compositions. However, when the concentration of boron is smaller than 10 atomic percent, the degree of amorphousness attained by the rapid quenching method lowers, so that there cannot be obtained a ribbon or thin sheet of an amorphous alloy having good quality and high mechanical strength. Thus, this ribbon is also not suitable as a sensor material.
The known amorphous magnetic alloys have been discussed with reference to oil pressure sensors, which discussion is true of torque sensors. In automobiles, torque sensors are ordinarily installed at the inside of or near a transmission, which is usually full of oil or oil mist. When instantaneous torque is applied to an amorphous magnetic alloy ribbon, a shear stress of 30 to 40 kg/mm.sup.2 is produced. Accordingly, it has been highly demanded to develop amorphous soft magnetic alloys which have good mechanical strength, corrosion resistance and magnetostrictive effect and which are inexpensive.